1. Field of the Invention
This invention relates to testing. Specifically, the present invention relates to testing fiber-optic cables.
2. Description of the Related Art
With the expansion of the Internet, wireless communications and conventional telephony, the need for communication services has increased. The increased need for communication services correlates to an increased need for communication infrastructure. Modern high bandwidth communication infrastructure is typically implemented with fiber-optic cable.
Fiber-optic cable provides high bandwidth communications. The cable itself typically consists of a glass-like core surrounded by several layers of protective material. Communication is accomplished in fiber-optic cable by transmitting light down the glass-like core. Information is modulated on the light and conveyed between transmission points. The light travels on material (e.g. core material) that serves as a transport medium. The transport medium has characteristic properties that can be measured and characterized. The glass-like core has to be properly matched with other pieces of fiber-optic cable and communications equipment. If the core is not properly matched light reflections occur disrupting the communications.
Fiber-optic cable deployed for commercial use may be damaged or have faults. For example, the glass-like core may crack or fracture under stress or when new pieces of fiber-optic cable are connected together, the light traveling down the glass-like core medium may be inappropriately reflected. Therefore in addition to the increase and focus on deploying fiber-optic cable, there is an increased interest in troubleshooting fiber-optic cable.
One noted approach for troubleshooting fiber cable uses an Optical Time Domain Reflectometer (OTDR). OTDR is used to evaluate the characteristic properties of fiber. An OTDR generates an initial light signal (e.g. reference trace) to characterize the fiber-optic cable. If the light hits any discontinuities (e.g. faults) such as cracks, fractures, bends, breaks in the cable, connection points to other cables, or connections to end electronics; the initial light signal reflects back (e.g. reflected signal) to the OTDR. The OTDR calculates the distance to the discontinuity in the fiber by measuring the time elapsed between transmission of the initial light signal and reception of the reflection.
In addition to reflected signals that result from faults, as light travels along fiber, it is attenuated by Raleigh scattering. This is caused by small changes in the index of refraction of the glass-like core. Some of the light scatters directly back to the OTDR. This effect is called back scattering. The back scattered light (e.g. reflected signal) is used as a means of characterizing the fiber-optic cable when there are no faults or when there are known faults such as connection points between fibers.
Generally speaking, a fault is anything that causes a loss in the reflection of the normal scattering of the fiber material. An OTDR displays the result of reference signals and test signals on a display. The vertical access of the display represents power in decibels and the horizontal access of the display represents distance.
During operation of an OTDR, the OTDR is connected to the fiber cable. The OTDR generates a reference trace to characterize the fiber. In addition the OTDR generates a test trace to test the fiber. The reference trace is compared with the test trace. An operator inputs and maintains threshold information and known fault information. For example, the threshold information defines the amount of difference that the operator is willing to accept, between a reference trace and a test trace, before he considers a discontinuity a fault. Known faults are points along a cable run where the operator expects to see a discontinuity such as a location where the fiber-optic cable is connected to other fiber-optic cables or equipment. Discontinuities from known faults produce a positive spike when the trace is graphed on the OTDR display. These spikes are often referred to as reflectance spikes.
Test traces may then be measured. The test traces works in the same way as a reference trace. An initial light signal is sent down the fiber-optic cable and a reflected signal comes back to the cable. The test trace is then compared to the reference trace. If the two traces are outside of a threshold limit, an operator is able to detect a possible fault. The operator will then go out to make a physical inspection to determine if the fault is an actual fault or a false alarm. For example, an operator may establish that a distance between the reference trace and the test trace of 3 db (e.g. known as a user-input threshold) is acceptable, however, a difference between the test trace and the reference trace of 4 db may be unacceptable (e.g. signal a fault).
In conventional OTDR systems both the reference trace and the test trace are displayed on a screen for the operator to review the two patterns. Using visual analysis, the operator attempts to detect and determine whether there is a discontinuity (e.g. fault) within the fiber-optic cable. Once a discontinuity is determined and identified, most OTDR systems also calculate the distance to the discontinuity by measuring the return time for the reflected signal.
In conventional OTDR systems fault analysis is traditionally done by sampling the loss of signal at various points along the length of fiber-optic cable. The loss levels are then compared to the corresponding points on a known good trace. When the difference between the two traces exceeds a certain threshold, a fault is expected to have occurred. However the threshold may be exceeded by many other factors, such as the insertion of equipment, degraded output of the OTDR light source, etc. As a result, in conventional OTDR systems there are a large number of false alarms. In addition, actual failures may be detected properly, but imprecisely located if the fault location does not correspond with the user-input threshold. Therefore the detection of faults requires manual oversight as well as the input and maintenance of threshold loss values which trigger an alarm. These values are arbitrarily chosen on a per cable basis and the values have to be updated whenever the cable and other parameters change.
An OTDR display is shown in FIG. 1. The OTDR display is a graph with the loss in decibels shown along the vertical axis and the distance in meters shown along the horizontal axis. A reference trace is shown as 100. The reference trace identifies the characteristic pattern (e.g. waveform) of the fiber-optic cable. A test trace may then be generated and compared to the reference trace to determine faults in the fiber. A test trace is shown as 102. The test trace is compared to the reference trace 100 to determine any faults in the fiber. As shown in FIG. 1, the reference trace and the test trace mirror each other until the fault location shown as 104. The fault location 104 would designate a fault. It should also be noted that a spike such as the spike shown as 106 is referred to in the art as a reflectance spike.
FIG. 2 displays a false alarm in a conventional OTDR system. A threshold is established for the difference between a reference trace and a test trace. If the threshold is exceeded, it is an indication to the operator that a fault exists in the fiber-optic cable. In FIG. 2, a reference trace is shown as 200. The reference trace 200 characterizes the fiber-optic cable. A test trace is shown at 202, the test trace mirrors the reference trace. A threshold is shown as 204. The threshold is a user-defined limit, above which an alarm is signaled which represents a fault in the fiber-optic cable. As shown in FIG. 2, the reference trace 200 and the test trace 202 are separated by more than the threshold shown as 204. However, this may be a false alarm. The difference in value may be caused by the insertion of equipment, degraded output of the OTDR light source, etc. However, there is no way for the operator to tell whether this is a false alarm or not, without making physical and manual inspections of the fiber-optic cable, at the point of the potential fault.
In addition, when new cable is attached to the fiber-optic cable or new equipment is connected to the fiber-optic cable a new reference trace must be produced and new-user defined limit may need to be established. Therefore there is a continual need to change and store new reference traces and update user-defined limits, as a result of the continuing development, redesign or reengineering of a network which includes the fiber-optic cable.
Thus there is a need for an OTDR system that reduces the amount of false alarms. There is a need for an OTDR system that provides improved fault location without manual intervention. There is a need for an OTDR system that automatically detects faults without a reference trace. Lastly, there is a need for an OTDR system that has less reliance on manual input, the maintenance of thresholds and the maintenance of performance parameters.
A method and apparatus for testing fiber-optic cable is presented. Signal information is collected using an Optical Time Domain Reflectometer (OTDR). The signal information includes information on the performance and characteristics of the fiber-optic cable. Characteristics such as scatter profile information, fault information, etc, is acquired. After the acquisition of the signal information, post acquisition processing is performed on the signal information. The post acquisition processing includes taking a derivative of the signal information, such as a first-order derivative of the signal information. The waveform detailing the results of the post acquisition processing is displayed for operator review. By reviewing the first-order derivative an operator is able to accurately locate and determine a fiber-optic fault.
In one embodiment of the present invention, a method of testing fiber-optic cable represented by a trace is presented. The trace includes reflectance spikes and a slope. The method comprises the steps of sampling the trace thereby producing sampled values; generating a first-order derivative of the sampled values thereby producing differential values including the reflectance spikes and the slope; removing the reflectance spikes from the differential values thereby producing sloped values including the slope; removing the slope from the sloped values thereby producing filtered values; and identifying a discontinuity in the filtered values thereby determining a fault. Further, in the method testing fiber-optic cables the step of removing the reflectance spikes includes the step of determining that the differential values have a positive slope. Lastly, in the method testing fiber-optic cables the step of removing the reflectance spikes includes the step of determining that the differential values exceed a trigger.
In another embodiment, a method of testing fiber-optic cable represented by a trace is presented. The method comprises the steps of generating a first-order derivative of the trace and determining a fault in response to the first-order derivative.
In an alternate embodiment, a method of testing comprises the steps of acquiring signal information from a fiber-optic cable, the signal information characterizing operation of the fiber-optic cable; determining a rate of change of the signal information; and determining a fault in the fiber-optic cable in response to the rate of change of the signal information.